This invention is directed towards transmitting data over telecommunications networks, and more specifically towards a system and method for adjusting the phase and/or symbol frequency of an analog modem""s transmitter.
Data transmission becomes more and more important as computer systems are used to support data intensive applications like the transfer of sounds, images, video and other media. Today""s telecommunications network is primarily digital. For example, the public switched telephone network (PSTN) is almost entirely digital. The only analog portions of the network are the subscriber loops that run to homes from telephone central office switching systems.
When analog data are transmitted over the telecommunications network, the codec equipment at the telephone central office (CO) samples and quantizes the analog signals traveling through the analog loops at a frequency of 8 kHz. This 8 kHz sampling rate standard is utilized throughout the entire digital portion of the telecommunications network. The sampling clock of the CO codec has a fixed frequency at 8000 samples/sec set by the network.
The rate at which the analog signal is sampled (the number of samples taken per second) is important because it determines the quality of the signal that is generated when the digital signal is converted back to analog form. The Nyquist theorem states that in order to accurately reconstruct an analog signal from its digital samples, the sampling rate used must be greater than or equal to two times the maximum frequency component present in the band limited signal. For example, if the maximum frequency component present in an analog signal is 250 kHz, the signal must be sampled at a minimum of 500 kHz in order to be able to recover the signal from its samples with minimal information loss.
Because of the Nyquist limit, the sampling rate used by telephone central office switching equipment (8 kHz) imposes a maximum frequency of 4 kHz on signals that can be passed through the telecommunications network from an analog loop. A bandwidth of 4 kHz provides for acceptable quality voice transmission, without requiring higher speed sampling requirements and equipment. However, for data transmission, such as from a modem, this bandwidth limit is problematic. The maximum frequency of signals that can be transmitted successfully in the upstream direction (through the telecommunications network from an analog subscriber loop) is 4 kHz. In the upstream direction, there is no extra bandwidth available because the codec""s sampling rate is locked at 8 kHz. Any signal energy that gets through the stop-band region of the 4 kHz low-pass filter is folded into the digitized signal according to a process known as xe2x80x9caliasingxe2x80x9d. Older modem protocols like international telecommunications union (ITU) V.90 and V.34 have very little excess bandwidth (energy outside the nominal band) in the upstream direction. V.34 supports much lower bit rates, and the trellis coded quadrature amplitude modulation (QAM) scheme it employs results in very little excess bandwidth.
In the downstream direction, this problem of the locked 8 kHz sampling rate does not exist. Because the analog to digital (A/D) converter employed by the downstream analog modem is not constrained to 8 kHz digitization, its sampling rate can be readily increased or decreased, within limits.
When rapidly transmitting data in the upstream direction using an ITU-V.92-like pulse code modulation (PCM) modem, a pre-equalizer must be employed by the analog modem transmitter to compensate for local loop channel distortion. When the sampling rate is below the Nyquist rate, the performance of a (pre)equalizer is severely affected by the fractional sampling phase offset of the received symbol stream relative to the codec clock. The effect can be large for symbol spaced equalizers operating on received analog signals with significant excess bandwidth. Because the telecommunication network sampling rate is fixed at 8 kHz, a digital modem operating on the telecommunication network employing a high-speed (wide bandwidth) PCM upstream modulation scheme falls into this category. The initial phase of the signal received at the digital modem (central office line-card codec) is determined by the random call timing of the analog modem and the loop channel. The analog modem digital-to-analog converter runs on a independent clock.
However, since the actual codec in use by the digital modem is locked to network timing and is not under the digital modem""s control, it is not possible for the digital modem to adjust the sampling phase of the upstream digitizer. Therefore, throughput of data can be degraded.
Another problem occurs from the separate clock signal of the analog modem. As previously described, the digital-to-analog converter of the analog modem runs on a clock independent of that of the network. The frequency for this clock is supposed to be 8000 samples/sec. However, depending on the type of crystal used in the analog modem, the frequency maybe slightly off. Since the digital modem must lock to the network timing, even slight differences in the frequency between the analog modem""s clock signal and the digital modem""s clock signal will result in data loss.
While this problem has been described in terms of telephony signals with analog loops, the same problem occurs in many signal transmission systems where excess bandwidth is received, but the receiver cannot change the sampling rate and/or phase of a codec (or other type of A/D converter).
A system and method for adjusting the frequency and/or phase of the analog signal produced by an analog modem connected to a digital modem over a telephone network. The digital portion of the telephone network is locked to the network clock, and the modems have no control over the sampling timing and/or rate as the analog signal is sampled and quantized by a codec. If distortion such as phase shift occurs to the analog signal, then the codec may be sampling at xe2x80x9cunresolvablexe2x80x9d transition points on the analog signal, thereby causing errors and a decrease in the usable bandwidth for transmitting data. The analog modem is locked in frequency to the clock of the digital network using loop-back timing. A phase estimate is computed using the quantized samples of a reference signal known to the digital modem. Next, a xe2x80x9cphase offsetxe2x80x9d is calculated by comparing the phase estimate to an optimum phase value. Then the digital modem sends the calculated phase offset information to the analog modem. The analog modem then delays or advances its transmitted signal by the phase offset. After the phase of the analog modem""s transmitter is adjusted, the analog signal reaches the codec at the phase desired by the digital modem. Alternatively, or in conjunction with the phase adjustment, the analog modem adjusts its frequency using the information learned by the timing recovery/tracking algorithm in the downstream direction (learns the network timing). Then the analog modem""s transmitter uses this timing for transmission.
According to one embodiment of the present invention, a method is provided for adjusting an analog signal produced by a transmitter and transmitted over an analog circuit, where the analog signal is received and converted to a digital signal locked to a fixed clock, and the digital signal is then received by a receiver. Steps include locking the transmitter frequency to the fixed clock, and adjusting a phase of the analog signal to align with the fixed clock, using information provided by the receiver. Adjusting the phase includes computing a phase estimate of the analog signal received, calculating a phase offset value to offset the phase of said analog signal received, and providing the phase offset value to the transmitter. The transmitter then adjusts the phase of the analog signal in accordance with the phase offset value.
The present invention includes an analog modem for transmitting data as an analog signal over an analog circuit, wherein said analog signal is received and converted to a digital signal by an A/D converter locked to a fixed clock. The digital signal is received by a digital modem. The analog modem includes a transmitter coupled to the analog circuit, the transmitter converting the data into the analog signal; a transmitter clock component, to provide a transmitter clock signal to the transmitter to allow the transmitter to convert the data into the analog signal; and a receiving component coupled to the analog circuit, the receiving component to receive information from the digital modem, the information including timing of the fixed clock. The transmitter clock component then adjusts the frequency and phase of the transmitter clock signal based on the information received by the receiving component. The transmitter clock component locks the transmitter clock signal to the fixed clock, and further adjusts the phase of the transmitter clock signal to align with the fixed clock.
An advantage of the present invention includes a lower error rate and/or an increase in the speed of data transmission during a connection by improving the performance of the pre-equalizer employed during data mode.
Another advantage of the present invention is that it allows a digital modem to direct a remote analog modem to adjust the relative phase of its transmitter.
Another advantage of the present invention is that it allows a digital modem to direct a remote analog modem to adjust the relative frequency of its transmitter.